A river environment with its structural and hydraulic features, creates habitats for aquatic life, of which fishes are the most notable. The river environment is especially indispensable for the reproduction of migrating fish as it offers them spawning and rearing areas. Transportation, water pollution, power production, and most of all dam construction have diminished these areas. Recently, the value of natural fish stocks has been more widely understood, and symposia have been held on the coexistence of hydraulic structures with the natural environment (Anon 1994, Leclerc et al. 1996). Improvement of the river environment is one way of providing for the future of naturally reproducing fish stocks. In addition, fishways can be used in improving natural fish migration.
Biologically speaking, fishways are special structures assisting upstream migrating fish in overcoming obstructions that block their way to spawning grounds, although the fishways can serve as routes for other purposes as well. Hydraulically speaking, fishways are energy dissipating structures. Fishway design has progressed from crude roughened channels into three main types – pool-and-weir, vertical slot, and Denil fishways. In addition, there are also other facilities, such as fish locks, culvert fishways and natural fishways. All of them can simply be regarded as water passages around or through an obstruction, designed to dissipate the energy of the flowing water to facilitate fish ascent.
A long history of fishways has been recorded. According to written notes, fishways were constructed at least as early as the 17th century in France (Katopodis & Rajaratnam 1983). These fishways consisted of steep, broad, open channels, the bottom of which were roughened with bundles of branches. Undoubtedly there were earlier attempts of even more primitive nature. However, the need for fishways increased around the 1850’s when hydraulic turbines and dam construction for water power production began to appear to a larger extent. In addition to dams and other man-made obstructions, fishways have been built in connection with natural obstructions. At natural obstructions, probably the earliest documented fishways – or fish ladders as they may also be called – were built in the River Ballisodare, in England in 1852-54 (Nordqvist 1892).
Early design efforts were based more on intuition than on scientific and engineering endeavor. Before the early 1900’s, fishway development included detailed plans but little scientific and engineering methodology. Probably the first scientific investigations of fishway hydraulics and design were carried out in Belgium by G. Denil in the beginning of the 1900’s (Orsborn 1987). He introduced a new fishway design that was very efficient in energy dissipation, the so-called Denil fishway (Katopodis & Rajaratnam 1983). His inspiration was most likely the design of Mr. M. MacDonald from Virginia, USA. Later, the originally complex structure was improved and recommendations for dimensions were given for the structure (McLeod & Nemenyi 1941, Committee on Fish-Passes 1942). The design G. Denil introduced was very advanced and an exceptionally good energy dissipator. Despite that, it can be said that the best thing Denil gave to the development of fishways was that his designs and studies stimulated research into fish behavior and the application of hydraulic engineering to fisheries’ problems (Orsborn 1987).
Research on fishway hydraulics began with a new earnest in the 1980’s. Especially Denil fishways have been a subject of intense hydraulic studies (e.g. Lonnebjerg 1980, Rajaratnam & Katopodis 1984, Rajaratnam et al. 1985, Rajaratnam et al. 1997, Kamula & Bärthel 2000). This is due to its characteristics: little space is required compared with other fishway types of the same water carrying capacity, and reasonably low water velocities. In addition to the Denil fishways, studies on the hydraulics of pool-and-weir fishways (e.g. Orsborn 1986, Rajaratnam et al. 1988, Boiten 1989, Boiten 1990, Bates 1991) and vertical slot fishways (e.g. Rajaratnam et al. 1986, Rajaratnam et al. 1992, Kamula 1995) have been carried out. Apart from the studies on special structures, considerations of different structures and their hydraulics, as well as their design criteria and new developments have been presented (e.g. Bell 1986, Rainey 1991, Vigneux 1992, Anon 1996).
For proper operation of a fishway, it is crucial that both biological and hydraulic aspects are considered. The response of fishes to fishways has been studied, in addition to fishway hydraulics (e.g. Jones et al. 1974, Slatick 1975, Lonnebjerg 1980, Slatick & Basham 1985, Swalme et al. 1985, Blackett 1987, Katopodis et al. 1988, Peake 1997, Laine et al. 1998). In the 1990’s, fishway research, construction and management started to gain more interest, and special conferences focusing on fishways have been held (e.g. Anon 1990, Anon 1995, Jungwirth et al. 1998, Kamula & Laine 1999).
Research for the sake of research is not a fruitful approach. In fishway studies, one main aim has always been to be able to design better operating fishways with sparing use of discharge and easiness of maintenance. Several design handbooks and guidelines for fishway design have been published in addition to research reports. In North America there is an outstanding interest on fish passage and related issues. In 1986, Milo C. Bell wrote a well-known handbook on fish passage design. Also, the American Fisheries Society (AFS) has held fish passage conferences in connection with its annual meetings. In addition to conference proceedings, the AFS has published a large summary on innovations in fish passage technology (Odeh 1999). In Germany, a handbook was published for fish passage design for European conditions (Anon 1996) and later translated into Finnish (Suomen Ympäristökeskus 1999).